A clearly pleased President Obama strode into the Rose Garden on Thursday to announce that a goal sought by the United States for decades — restrictions on Iran's nuclear program — could be at hand if a final accord is hammered out.

“This framework would cut off every pathway that Iran could take to develop a nuclear weapon," Obama said. "Iran will face strict limitations on its program, and Iran has also agreed to the most robust and intrusive inspections and transparency regime ever negotiated for any nuclear program in history.”

The president detailed restrictions on the atomic materials Iran will be able to retain and the locations where they can be held. He repeatedly stressed that the accord would be based on verification, not trust.

“If Iran cheats, the world will know it. If we see something suspicious, we will inspect it," Obama said. "Iran’s past efforts to weaponize its program will be addressed. With this deal, Iran will face more inspections than any other country in the world.”

Recent polls showed Americans open to a negotiated settlement of the nuclear standoff with Iran, but that's not the case with all U.S. lawmakers. Last month, 47 Republican senators sent an open letter to Iran promising a de facto congressional veto of any deal reached.

After Thursday’s announcement, the letter’s author, Senator Tom Cotton of Arkansas, dismissed the accord as “a list of dangerous U.S. concessions that will put Iran on the path to nuclear weapons.”

In a statement, House Speaker John Boehner of Ohio said, “Congress must be allowed to fully review the details of any agreement before any sanctions are lifted.”

Obama welcomed congressional debate but sought to inoculate the deal from harsh criticism.

“When you hear the inevitable critics of the deal sound off, ask them a simple question: Do you really think that this verifiable deal, if fully implemented, backed by the world’s major powers, is a worse option than the risk of another war in the Middle East?" he said. "Is it worse than doing what we’ve done for almost two decades, with Iran moving forward with its nuclear program and without robust inspections?”

Nuclear expert James Acton of the Carnegie Endowment said that "if legislators needed proof of concept that a stringent deal that restricts Iran’s nuclear program is reachable, they should have that evidence in today’s deal.”

Acton had been a skeptic of the negotiations but seemed pleasantly surprised by the outcome.

“I am not going to predict what legislators will do," he said. "We all recognize that legislators are often motivated by political considerations rather than by the text of the agreement. If they judge this agreement on its merits, this agreement deserves their support.”

The president stressed that the accord would not erase decades of mistrust between Washington and Tehran. But at a time of galloping conflicts from Ukraine to Syria to Yemen that have repeatedly put the administration’s foreign policy to the test, Obama, a Nobel Peace laureate, is within striking distance of a historic accord that, if completed and successful, could cement his legacy on the world stage.